American middle distance runner Maggie Vessey has accepted a public warning from the United States Anti-Doping Agency (USADA) for testing positive for a diuretic at the IAAF World Challenge meeting in Zagreb, Croatia, on September 4. The substance, Canrenone, an aldosterone antagonist, was present in a prescription medication Vessey was taking for her skin.

“I took a prescription skincare product that was prescribed to me by my family practitioner, that I did not know contained a diuretic,” Vessey said in a statement provided by USADA. “As soon as I was notified of my positive test, I cooperated with USADA and provided them everything they asked for in order to demonstrate that I made an honest mistake, and that the medication did not enhance my performance in any way.”

At the meet in Zagreb, Vessey won the 800m in 1:59.61. She will be disqualified from that meet, however, and must forfeit any earnings. That elevates her New Balance teammate Brenda Martinez to first place who clocked 1:59.63 behind Vessey.

Diuretics are banned primarily because they allow athletes to excrete banned substances more quickly, thus acting as a masking agent for the use of other performance enhancing drugs. Vessey, however, will be allowed to continue to use the skin product under a doctor’s supervision by applying for a special exemption for “therapeutic use.”

“I have since applied for a Therapeutic Use Exemption for this medication,” Vessey added. “I share in USADA’s belief in clean sport, and look forward to the upcoming track season.”

Vessey, 31, enjoyed her best season in 2011 in the 800 meters when she finished sixth at the IAAF World Championships, took second at the USA Championships, and won the Mt. SAC Relays. Her career best time is 1:57.84 set in 2009 in Monaco.